Considering that a 'positive' quad screen means that you automatically get a level 2 ultrasound...and given that the screen is solely based on placental hormones (which change with age--that's why there's a time limit on the test BOTH ways, too early and too late...and why it gets screwed up with multiples), I wouldn't bother.
If you are truly worried about chromosomal abnormalities (or structural ones, for that matter) I highly highly highly suggest a level 2 u/s. Yes, it is more expensive, but insurance is likely to cover it (or...if it needs to be indicated, take the screening test, which is sure to be abnormal, and that'll clear the way). Keep in mind, though, that many 'markers' of abnormalities that the tech is looking for DO NOT indicate for sure that there's anything wrong. Only an amnio can do that for sure.
I know a vomitrocious amount about screening tests/processes/ultrasounds/crap like that even though I'm not an medical professional. (It's a long story, relating to my twins, but I won't bore/frighten anyone with it here) Feel free to PM me if you want.
But mama, if you are seriously worried, please give some thought to the ultrasound and then jump through the hoops (like the screening test) if you need to.